Ferroic quadrupolar ordering in CeCoSi revealed using Co59 NMR measurements

A nonmagnetic phase transition at ${T}₀\ensuremath{\sim}12$ K in the tetragonal system CeCoSi with a Kramers doublet ground state is reminiscent of an electric quadrupole ordering, even though its well-separated crystal-electric-field (CEF) levels are unlikely to acquire higher-order multipole degrees of freedom. Here, we report $⁵⁹\mathrm{Co}$ nuclear magnetic resonance (NMR) studies that are highly compatible with a ferroic quadrupole ordering below ${T}₀$. Changes in the NMR spectra below ${T}₀$ suggest that an external magnetic field induces ferroic Ce dipole moments orthogonal to the field, enabling domain selection in the nonmagnetic phase. Our findings suggest the presence of a ferroic ${O}_zx$-type quadrupole component in CeCoSi and demonstrate that quadrupole ordering may occur under well-separated CEF levels in tetragonal systems.